Traditional methods of wastewater treatment involve bringing wastewater streams into contact with bacteria either in an aerobic or anaerobic type process in what is known as activated sludge treatment. These bacteria consume parts of the substrate material or waste contained in the wastewater, which are typically organic compounds containing carbon, nitrogen, phosphorus, sulfur, and the like. Typically, a portion of the waste is consumed to further the metabolism of the bacterial cells or maintain the physiological functioning of the bacterial cells. In addition, a portion of the waste is also consumed as part of the process of synthesis of new bacterial cells. The activated sludge treatment process yields a certain amount of sludge and associated solids which must be continuously removed from the treatment basin to maintain the steady state sludge balance which is critical to the effective functioning of the activated sludge treatment system.
In order to maintain waste removal capacity of the treatment plant at steady state it is important to control the generation of new bacterial cells within the activated sludge treatment process. Too much synthesis of new bacterial cells in excess of what is required for the treatment of the waste at or near steady state results in excess biosolids formation attributable to the accumulation of such newly synthesized but unneeded bacterial cells. This excess biosolids must be continuously removed during the activated sludge treatment process.
Existing methods for dealing with the removal of sludge includes transporting the sludge to landfills, utilization of sludge for land application or agricultural purposes, and incineration of the sludge. Most sludge disposal operations require some prior treatment of the sludge; a process known in the art as solids handling. Solids handling processes are often costly and time consuming operations and typically involve one or more of the following steps: (a) the concentration of the sludge in a thickener, usually requiring the use of polymers; (b) digestion of the sludge in order to stabilize the bacteria and to further reduce the volume and pathogen content of the sludge; (c) dewatering of the sludge to reach approx 15-25% solids content; which involves the passage of the sludge through centrifuges or other solid-liquid separation type devices; (d) storage of the sludge; and (e) transportation to sites for landfill, land application by farmers, or other end use.
It is estimated that the costs associated with solids handling and disposal processes can be between 20-60% of total operating costs associated with the overall wastewater treatment process. Due to the cost and time associated with solids handling and disposal, it is beneficial to minimize the amount of excess sludge produced in the wastewater treatment process.
In conventional activated sludge treatment systems and methods, oxygen is required both for the chemical oxidation of the substrate material (i.e. waste) as well as for the synthesis of new cells and metabolic processes of the bacterial cells. Use of ozone in addition to oxygen for the treatment of sludge has also been reported. More particularly, ozone treatment of sludge has been reported in combination with mechanical agitators and/or a pump providing the motive mixing. The sludge-ozone contact typically occurs in a continuously stirred tank reaction (CSTR) mode, and lysis (breaching of the integrity of the cell wall) results as a consequence of the strong oxidizing action of ozone on the cell walls. Lysis leads to the release of the substrate rich cellular content of the bacterial cells. In this way, the solid cells which would otherwise have been discharged as excess sludge are lysed, and by so doing, they are transformed to substrate which can then be consumed by bacteria in the treatment basin.
The cellular content is a liquid matrix which is comprised of proteins, lipids, polysaccharides and other sugars, DNA, RNA and organic ions. Because of the low selectivity that occurs when sludge ozone contacting is carried out in a continuously stirred reactor mode, excessive amounts of ozone are consumed using prior methods for sludge ozonation. In addition, some prior reported uses of ozone required specialized pre-treatment or modification of the sludge. Such pre-treatments and modifications may include adjusting the pH of the sludge, increasing the temperature of the sludge, increasing the pressure of the ozone treatment vessel, or passing the sludge through anaerobic pre-digestion steps. Thus, the prior use of ozone in the treatment of sludge involved additional complexity, materials, equipment and the increased costs associated therewith.
Three major methods for reactor systems are known, these being the Continuously Stirred Tank Reactor system (CSTR), the higher selective Plug Flow Reactor (PFR) and the Batch Reactor System (BRS). The major difference between the different reactor modes lies fundamentally in: (i) the average amount of time that a molecule stays within the reaction space, also known as the residence time; (ii) the interaction between reacting ‘parcels’ e.g., there is significant back-mixing in the CSTR, while the PFR is characterized by very limited, if any, back-mixing; and (iii) the yield obtained.
In addition to the challenges associated with sludge reduction, one of the other formidable challenges in many wastewater treatment operations is the control of foaming and bulking in the wastewater. Foaming and bulking problems are generally caused by the presence of an abundance of filamentous bacteria such as Nocardia and Parvicella bacteria in the wastewater. Such filamentous organisms tend to prosper in wastewater where there is either a low level of dissolved oxygen in the wastewater or a deficiency of nutrients in the wastewater as characterized by a low food to microorganism ratio.
Foaming problems are often characterized by the presence of a light tan foam covering up to 25% of the activated sludge basin surface and often contribute to operational problems within the wastewater plant including undesirable odors, solids washout and generally hazardous working conditions, Bulking, on the other hand, is characterized by the presence of filamentous organisms that extend from the floc thereby causing an interference with settlement and compaction properties of sludge. This adversely impacts clarifier performance and often results in poor effluent quality.
The control of bulking and foaming problems in wastewater treatment operations requires the elimination or control of filamentous organisms in wastewater. Conventionally, the control of filamentous organisms in wastewater is effected through the addition of oxidizing agents like chlorine, peroxide and ozone to the activated sludge basin. Alternate means of controlling of filamentous organisms include increasing the nutrient loading or improving the oxic state of the sludge or both.